The letter X has a big reputation. It’s used to designate things that are powerful, scary, mysterious, or all of the above. So we have X-rays and X Men, the X Files and X planes, and in the movies, at least a couple of spies designated triple-X.

The Sun gets into the act, too, with X-class solar flares — eruptions that are powerful, scary, and a bit mysterious.

Solar flares are massive explosions on the Sun’s surface. They occur when the lines of the Sun’s magnetic field intertwine and then snap. They generate outbursts of particles and radiation that race into the solar system — and that can have a dramatic impact on Earth.

Scientists use five letters to designate the power of a solar flare. From weakest to strongest, they’re A, B, C, M, and X.

The first three categories are fairly puny. M-class flares can have some impact on Earth, but the really nasty ones are the X-class. The largest can produce as much energy as a billion hydrogen bombs.

They’re often accompanied by massive outbursts of charged particles. The combination can fry orbiting satellites, disrupt radio broadcasts, and knock out power grids over large areas. They can also force airlines to reroute flights away from the north pole to avoid dangerous levels of radiation. And they can produce displays of the northern lights in places that seldom see them — places like Florida, Texas, and California — colorful curtains of light powered by the X factor.